Reference manual: fix typo in doc of [tryif/then/else].
Reference manual: fix typo in doc of [tryif/then/else].

Fixes #3939.
JasonGross/coq,8l/coq,SkySkimmer/coq,barras/coq,asr/coq,letouzey/coq-wip,mlasson/coq,TOSPIO/coq,jpdeplaix/coq,ppedrot/coq,herbelin/github-coq,ejgallego/coq,ppedrot/coq,SkySkimmer/coq,ejgallego/coq,gares/coq,Matafou/coq,letouzey/coq-wip,coq/coq,william-lawvere/coq,maximedenes/coq,coq/coq,letouzey/coq-wip,gmalecha/coq,barras/coq,TheTypoMaster/coq,aspiwack/coq,ejgallego/coq,JasonGross/coq,Matafou/coq,aspiwack/coq,Ptival/coq,SkySkimmer/coq,silene/coq,ppedrot/coq,JasonGross/coq,TOSPIO/coq,asr/coq,Ptival/coq,gares/coq,Lionel-Rieg/coq,TOSPIO/coq,letouzey/coq-wip,mlasson/coq,gares/coq,Lionel-Rieg/coq,ale-f/coq,mlasson/coq,Zimmi48/coq,amintimany/coq,Zimmi48/coq,TheTypoMaster/coq,ejgallego/coq,Zimmi48/coq,8l/coq,coq/coq,letouzey/coq-wip,william-lawvere/coq,asr/coq,maximedenes/coq,jpdeplaix/coq,ppedrot/coq,mattam82/coq,Lionel-Rieg/coq,amintimany/coq,william-lawvere/coq,silene/coq,mattam82/coq,maximedenes/coq,herbelin/github-coq,mattam82/coq,gares/coq,Matafou/coq,ppedrot/coq,gmalecha/coq,coq/coq,SkySkimmer/coq,maximedenes/coq,JasonGross/coq,amintimany/coq,barras/coq,Ptival/coq,ale-f/coq,8l/coq,maximedenes/coq,gares/coq,herbelin/github-coq,aspiwack/coq,Zimmi48/coq,silene/coq,gmalecha/coq,ale-f/coq,jpdeplaix/coq,TheTypoMaster/coq