Improve wording for slice in language reference
Improve wording for slice in language reference

ihc/futhark,diku-dk/futhark,diku-dk/futhark,ihc/futhark,diku-dk/futhark,HIPERFIT/futhark,diku-dk/futhark,ihc/futhark,HIPERFIT/futhark,HIPERFIT/futhark,diku-dk/futhark