Consistent use of <pre class="code">

Issue: SPR-8108

spring-expression/src/main/java/org/springframework/expression/ExpressionParser.java

@@ -29,7 +29,7 @@ public interface ExpressionParser {
 	/**
 	 * Parse the expression string and return an Expression object you can use for repeated evaluation.
 	 * <p>Some examples:
-	 * <pre>
+	 * <pre class="code">
 	 *     3 + 4
 	 *     name.firstName
 	 * </pre>
@@ -42,7 +42,7 @@ public interface ExpressionParser {
 	/**
 	 * Parse the expression string and return an Expression object you can use for repeated evaluation.
 	 * <p>Some examples:
-	 * <pre>
+	 * <pre class="code">
 	 *     3 + 4
 	 *     name.firstName
 	 * </pre>

spring-expression/src/main/java/org/springframework/expression/ParserContext.java

@@ -29,7 +29,7 @@ public interface ParserContext {
 	 * Whether or not the expression being parsed is a template. A template expression consists of literal text that can
 	 * be mixed with evaluatable blocks. Some examples:
 	 *
-	 * <pre>
+	 * <pre class="code">
 	 * 	   Some literal text
 	 *     Hello #{name.firstName}!
 	 *     #{3 + 4}

spring-expression/src/main/java/org/springframework/expression/spel/SpelMessage.java

@@ -25,7 +25,7 @@ import java.text.MessageFormat;
  * <p>
  * When a message is formatted, it will have this kind of form
  *
- * <pre>
+ * <pre class="code">
  * EL1004E: (pos 34): Type cannot be found 'String'
  * </pre>
  *