[ump45nose]

Overview:

Two documentation text fixes, no code changes:

• TROUBLESHOOTING.md:23 — the tl;dr line opens a bold span with ** but closes it with a single *:

  **tl;dr* `export GO111MODULE=on`
  

  The emphasis run is mismatched: it is opened with a two-character ** delimiter run and closed
  with a one-character *, so the line does not render as the intended bold label. Every other
  tl;dr line in the docs uses the correct form (README.md:716 and README.md:744 both say
  **tl;dr:** Yes. / **tl;dr:** No.), so this PR aligns line 23 with them:
  **tl;dr:** export GO111MODULE=on``.

• README.md:164 — "As a rule of the thumb" should be the idiom "As a rule of thumb".

What problem does it solve?:

The broken emphasis makes the one-line answer to the most-Googled Viper installation error look
like a markup mistake in rendered output, and the README sentence reads as a translation artifact.
Both are pure prose fixes; nothing in the API or behaviour changes.

Closes: N/A — no open issue covers these two lines.

Special notes for a reviewer:

• I deliberately left viper.go:201 ("By default it's value is "."") alone even though it is the
  same class of typo: PR fix: normalize key delimiter to lowercase for nested lookups (#2069) #2175 changes the line right below it inside the same hunk context, so
  touching it here would needlessly conflict.
• Overlap check on README.md: the open PRs that edit the file (docs: clarify extensionless config handling and SetConfigType usage #2155, docs: explain custom configuration file searches #2172, docs: fix typos in README.md and UPGRADE.md #2150, docs: update supported formats list for v1.20 changes #2102, Update README to clarify HCL, INI, and Java Properties support moved to plugin #2098)
  all land in the "Reading Config Files"/"Setting Overrides" regions (lines 92–146 and 220–226), so
  none of them edits line 164. On TROUBLESHOOTING.md, docs: point TROUBLESHOOTING at the maintained mapstructure fork #2162 edits line 5 and docs: TROUBLESHOOTING points at a viper_yaml3 build tag that is gone #2168 edits line 32,
  neither of which is line 23.
• Tests: documentation-only change; make test / go test ./... was not run in my sandbox, and
  markdownlint is not configured in this repo (.golangci.yaml lints Go code, .yamllint.yaml
  covers YAML). I verified the broken delimiter run and the two correct
  tl;dr lines already in README.md by reading upstream master.
• Prepared with AI assistance; I read and verified each cited line against upstream master
  (528f741) myself.