Improve Throwables.propagate javadoc
Improve Throwables.propagate javadoc

The mention of the type system is misleading, and a word was missing.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=105980357

mosoft521/guava,DavesMan/guava,typetools/guava,typetools/guava,google/guava,jakubmalek/guava,Xaerxess/guava,tli2/guava,GitHub4Lgfei/guava,aiyanbo/guava,tli2/guava,EdwardLee03/guava,rgoldberg/guava,xasx/guava,mosoft521/guava,EdwardLee03/guava,berndhopp/guava,mosoft521/guava,GitHub4Lgfei/guava,berndhopp/guava,DavesMan/guava,Xaerxess/guava,google/guava,rgoldberg/guava,google/guava,aiyanbo/guava,jakubmalek/guava,xasx/guava