base: Escape \ in CallStack example
base: Escape \ in CallStack example

[skip ci]

sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc