doc: correct supported gdb version info

GDB 7.5 recognizes DWARF4 by default.

GDB 7.5 release note does not explicitly mention DWARF4 support
but according to GCC 4.8 release note
 https://gcc.gnu.org/gcc-4.8/changes.html

 "DWARF4 is now the default when generating DWARF debug
 information. ...
 GDB 7.5, Valgrind 3.8.0 and elfutils 0.154 debug information
 consumers support DWARF4 by default."

Change-Id: I56b011c7c38fbc103bbd366ceaea3b709c66ab7f
Reviewed-on: https://go-review.googlesource.com/77570Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>

doc/debugging_with_gdb.html

@@ -32,7 +32,7 @@ In time, a more Go-centric debugging architecture may be required.
 <p>
 When you compile and link your Go programs with the <code>gc</code> toolchain
 on Linux, Mac OS X, FreeBSD or NetBSD, the resulting binaries contain DWARFv4
-debugging information that recent versions (&gt;7.5) of the GDB debugger can
+debugging information that recent versions (&ge;7.5) of the GDB debugger can
 use to inspect a live process or a core dump.
 </p>