Updates &elastic#42; characters to an escaped * (elastic#22269)

GitHub's asciidoc parser handles `*` characters fine but markdown thinks its italicizing unless you escape the first asterisk. Subsequent asterisks on the same line can stay unescaped and should work fine.

docs/settings/apm-settings.asciidoc

@@ -17,12 +17,12 @@ xpack.apm.enabled:: Set to `false` to disabled the APM plugin {kib}. Defaults to
 xpack.apm.ui.enabled:: Set to `false` to hide the APM plugin {kib} from the menu. Defaults to
 `true`.
 
-apm_oss.indexPattern:: Index pattern is used for integrations with Machine Learning and Kuery Bar. It must match all apm indices. Defaults to `apm-*`.
+apm_oss.indexPattern:: Index pattern is used for integrations with Machine Learning and Kuery Bar. It must match all apm indices. Defaults to `apm-*`.
 
-apm_oss.errorIndices:: Matcher for indices containing error documents. Defaults to `apm-*-error-*`.
+apm_oss.errorIndices:: Matcher for indices containing error documents. Defaults to `apm-\*-error-*`.
 
-apm_oss.onboardingIndices:: Matcher for indices containing onboarding documents. Defaults to `apm-*-onboarding-*`.
+apm_oss.onboardingIndices:: Matcher for indices containing onboarding documents. Defaults to `apm-\*-onboarding-*`.
 
-apm_oss.spanIndices:: Matcher for indices containing span documents. Defaults to `apm-*-span-*`.
+apm_oss.spanIndices:: Matcher for indices containing span documents. Defaults to `apm-\*-span-*`.
 
-apm_oss.transactionIndices:: Matcher for indices containing transaction documents. Defaults to `apm-*-transaction-*`.
+apm_oss.transactionIndices:: Matcher for indices containing transaction documents. Defaults to `apm-\*-transaction-*`.