doc: go1.12: update notes on go directive

Fixes #30043

Change-Id: I4ecfff7d8a9432240c1927f7484786fe1182b773
Reviewed-on: https://go-review.googlesource.com/c/160797Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.12.html

@@ -181,10 +181,10 @@ tour
 
 <p><!-- CL 147282, 147281 -->
   The <code>go</code> directive in a <code>go.mod</code> file now indicates the
-  version of the language used by the files within that module, and
-  <code>go</code> <code>mod</code> <code>tidy</code> sets it to the
-  current release (<code>go</code> <code>1.12</code>) if no existing
-  version is present.
+  version of the language used by the files within that module.
+  It will be set to the current release
+  (<code>go</code> <code>1.12</code>) if no existing version is
+  present.
   If the <code>go</code> directive for a module specifies a
   version <em>newer</em> than the toolchain in use, the <code>go</code> command
   will attempt to build the packages regardless, and will note the mismatch only if