[syntax-highlighting] Bump external repository to get bug fixes

ad522a0 driver: Fix printing error message to file when cannot resolve highlighters directory location
73cb832 'main': Highlight mismatched 'if'/'fi'.
be083d7 driver: Improve «unhandled ZLE widget 'foo'» error message.
237f89a 'main': Don't consider «$*» a glob.
835fec7 workaround for PAT_STATIC bug in zsh
9523d6d tests: zsh 5.4-to-be compatibility: Set a new "I am shooting myself in the foot" option.
aac4a44 driver: Fix duplicated slash in error message
4f49c4a docs: Update zplug install instruction
5efd062 tests: Add a regression test for issue sorin-ionescu#392 (aliases beginning with a '+' are lost).
67be621 tests: Move some code in preparation for next commit. No functional change.
74949c2 driver: Don't undefine aliases that begin with a '+', to workaround an upstream bug.
8d5afe4 driver: Be immune to 'alias' having been redefined.
76ea9e1 'main': Highlight possible history expansions in double-quoted strings.
50fbb5f docs: Update Homebrew link.
2dce602 driver: Be immune to weird aliases in the calling scope.
347cf0e 'main': Add regression test for previous commit.
5625e30 'main': Fix bug: no start_pos=$end_pos in comment short path
fed37a9 'main': Fix a bug concerning command word with embedded colon-space sequences.
626c034 Add FreeBSD port
3d74aa4 Add Fedora package
5398949 changelog: Update for changes pulled out of 0.5.x.