Fix perftools/php-profiler link

[kornrunner]

Changed syntax as [perftools/php-profiler] alias was already defined in the file.

[glensc]

Pardon, but what is the problem you are trying to solve here? and I prefer markdown reflinks for readability. Describe the problem first.

[kornrunner]

Both [perftools/php-profiler] anchors that are rendered on Github repository page (Readme.md) actually link to https://github.com/perftools/xhgui#profiling-a-web-request-or-cli-script which is the value of first markdown reflink.

It seems that Github doesn't process markdown reflink redefinition - thus second link is not linking to the repository page as one might expect (the one being changed within this PR).

[glensc]

perhaps the first link should link to the section in the readme, i.e:

[bla bla bla](#profiling-a-web-request-or-cli-script)

[kornrunner]

Updated first link