Fix typo in Span.md
Fix typo in Span.md

perfromance -> performance
VSadov/corefxlab,ericstj/corefxlab,stephentoub/corefxlab,adamsitnik/corefxlab,benaadams/corefxlab,ericstj/corefxlab,Vedin/corefxlab,whoisj/corefxlab,Vedin/corefxlab,weshaggard/corefxlab,ericstj/corefxlab,Vedin/corefxlab,ahsonkhan/corefxlab,dotnet/corefxlab,stephentoub/corefxlab,tarekgh/corefxlab,hanblee/corefxlab,stephentoub/corefxlab,VSadov/corefxlab,joshfree/corefxlab,joshfree/corefxlab,Vedin/corefxlab,VSadov/corefxlab,dotnet/corefxlab,alexperovich/corefxlab,tarekgh/corefxlab,stephentoub/corefxlab,ericstj/corefxlab,whoisj/corefxlab,benaadams/corefxlab,KrzysztofCwalina/corefxlab,stephentoub/corefxlab,ahsonkhan/corefxlab,VSadov/corefxlab,ericstj/corefxlab,hanblee/corefxlab,Vedin/corefxlab,stephentoub/corefxlab,adamsitnik/corefxlab,KrzysztofCwalina/corefxlab,VSadov/corefxlab,VSadov/corefxlab,ericstj/corefxlab,Vedin/corefxlab