regexp: remove reference to external RE2 wiki from the docs

[mauri870]

The regexp docs point to the google/re2 syntax page, which
causes confusion since users expect feature parity with RE2, which might
not be the case if you are using an older version of Go.

In regexp/syntax/doc.go there is already an autogenerated syntax based
on RE2 and we already link to that in the docs, so removing the external
link does not result in any loss of information.

Fixes #64108

[gopherbot]

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[gopherbot]

Message from [email protected]:

Patch Set 1:

(1 comment)

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