Fix RegEx for grep type

[n-thumann]

This PR fixes compatibility issues with grep on macOS.
Because macOS uses grep (BSD grep, GNU compatible) 2.6.0-FreeBSD, the RegEx for detecting the version doesn't work as expected.
I therefore removed the matching of the closing bracket ()) and added this different version string as a comment.

Before:

# vgrep -w "Homebrew" --no-less .          
panic: runtime error: index out of range [1] with length 0

goroutine 1 [running]:
main.(*vgrep).getGrepType(0x6542da0)
        /private/tmp/vgrep-20211031-92807-9uidb/vgrep-2.5.3/vgrep.go:264 +0xc8
main.(*vgrep).grep(0xc0000f8000, {0xc00007a040, 0x3, 0x4})
        /private/tmp/vgrep-20211031-92807-9uidb/vgrep-2.5.3/vgrep.go:304 +0x43b
main.main()
        /private/tmp/vgrep-20211031-92807-9uidb/vgrep-2.5.3/vgrep.go:184 +0x765

After (expected result):

# vgrep -w "Homebrew" --no-less .
Index File         Line Content
    0 ./test.txt:1    1 Hello from Homebrew

[vrothberg]

LGTM, thanks a lot for contributing!