Fix a typo

Fixes https://github.com/google/error-prone/issues/1665

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=314431923

core/src/main/java/com/google/errorprone/bugpatterns/PublicConstructorForAbstractClass.java

diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/PublicConstructorForAbstractClass.java b/core/src/main/java/com/google/errorprone/bugpatterns/PublicConstructorForAbstractClass.java
index d4573af..4098e4d 100644
--- a/core/src/main/java/com/google/errorprone/bugpatterns/PublicConstructorForAbstractClass.java
+++ b/core/src/main/java/com/google/errorprone/bugpatterns/PublicConstructorForAbstractClass.java
@@ -40,7 +40,7 @@
 @BugPattern(
     name = "PublicConstructorForAbstractClass",
     summary =
-        "Constructors of on an abstract class can be declared protected as there is never a need"
+        "Constructors of an abstract class can be declared protected as there is never a need"
             + " for them to be public",
     explanation =
         "Abstract classes' constructors are only ever called by subclasses, never directly by"