Fixes incompatability with gawk

Need to escape the '\' as well otherwise it will fail with gawk:

service carbon-cache restart
awk: cmd. line:1: warning: escape sequence `\[' treated as plain `['
awk: cmd. line:1: (FILENAME=/opt/graphite/conf/carbon.conf FNR=3) fatal: Unmatched [ or [^: /[cache:?/
awk: cmd. line:1: warning: escape sequence `\[' treated as plain `['
awk: cmd. line:1: (FILENAME=/opt/graphite/conf/carbon.conf FNR=3) fatal: Unmatched [ or [^: /[cache:?/
awk: cmd. line:1: warning: escape sequence `\[' treated as plain `['
awk: cmd. line:1: (FILENAME=/opt/graphite/conf/carbon.conf FNR=3) fatal: Unmatched [ or [^: /[cache:?/