make usage match script filename

stackcollapse-xdebug.php

@@ -30,14 +30,14 @@
 
 function usage($exit = 0) {
     echo <<<EOT
-stackcollapse-php.php  collapse php function traces into single lines.
+stackcollapse-xdebug.php  collapse php function traces into single lines.
 
 Parses php samples generated by xdebug with xdebug.trace_format = 1
 and outputs stacks as single lines, with methods separated by semicolons,
 and then a space and an occurrence count. For use with flamegraph.pl.
 See https://github.com/brendangregg/FlameGraph.
 
-USAGE: ./stackcollapse-php.php [OPTIONS] infile > outfile
+USAGE: ./stackcollapse-xdebug.php [OPTIONS] infile > outfile
     -h --help    Show this message
     -t           Weight stack counts by duration using the time index in the trace (default)
     -c           Invocation counts only. Simply count stacks in the trace and sum duplicates, don't weight by duration.