Typo fixed: Nullabe -> Nullable

Fixes #1077

RELNOTES: N/A

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=207278809

check_api/src/main/java/com/google/errorprone/dataflow/nullnesspropagation/TrustingNullnessAnalysis.java

@@ -37,7 +37,7 @@
 import org.checkerframework.dataflow.cfg.UnderlyingAST;
 
 /**
- * An interface to the "trusting" nullness analysis. This variant "trusts" {@code Nullabe}
+ * An interface to the "trusting" nullness analysis. This variant "trusts" {@code Nullable}
  * annotations, similar to how a modular nullness checker like the checkerframework's would, meaning
  * method parameters, fields, and method returns are assumed {@link Nullness#NULLABLE} only if
  * annotated so.