Documentation: change -L:<regex> to -L:<funcname>

Documentation: change -L:<regex> to -L:<funcname>

The old wording was somehow implying that <start> and <end> were not
regular expressions. Also, the common case is to use a plain function
name here so <funcname> makes sense (the fact that it is a regular
expression is documented in line-range-format.txt).

Signed-off-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>