Fix an example value using TagValueExpressionResolver

[giger85]

Refer to SpanTagAnnotationHandlerTests.java, @SpanTag(key = "test", expression = "'hello' + ' characters'") produces hello characters value of tag.

spring-cloud-sleuth/tests/common/src/main/java/org/springframework/cloud/sleuth/instrument/annotation/SpanTagAnnotationHandlerTests.java

Line 74 in 2b17d08

assertThat(resolvedValue).isEqualTo("hello characters");

So, hello characters is right value of tag in Resolving Expressions for a Value document section.