[gdb/doc] Fix qIsAddressTagged anchor

When building gdb with an older makeinfo (4.13), I run into:
...
gdb/doc/gdb.texinfo:44159: @anchor expected braces.
gdb/doc/gdb.texinfo:44159: ` {qIsAddressTagged}
...

This is related to this line:
...
@anchor {qIsAddressTagged}
...

Fix this by removing the space before the left brace.

Tested by rebuilding the documentation.