Updating issue templates to avoid @rustbot triggers

Author: xFrednet

.github/ISSUE_TEMPLATE/blank_issue.md

@@ -8,7 +8,7 @@ about: Create a blank issue.
 Additional labels can be added to this issue by including the following command
 (without the space after the @ symbol):
 
-`@rustbot label +<label>`
+@ rustbot label +<label>
 
 Common labels for this issue type are:
 * C-an-interesting-project

.github/ISSUE_TEMPLATE/bug_report.md

@@ -36,7 +36,7 @@ LLVM version: 10.0
 Additional labels can be added to this issue by including the following command
 (without the space after the @ symbol):
 
-`@rustbot label +<label>`
+@ rustbot label +<label>
 
 Common labels for this issue type are:
 * `I-suggestion-causes-error`

.github/ISSUE_TEMPLATE/false_positive.md

@@ -37,7 +37,7 @@ LLVM version: 10.0
 Additional labels can be added to this issue by including the following command
 (without the space after the @ symbol):
 
-`@rustbot label +<label>`
+@ rustbot label +<label>
 
 Common labels for this issue type are:
 * I-suggestion-causes-error