docs: fix/improve the structure of several documents

Change-Id: Ib8acd4bf29baf19b78d25cabc1a873eeeec15da6
diff --git a/docs/specs/signed-interest.rst b/docs/specs/signed-interest.rst
index 67a88b3..0ecb72b 100644
--- a/docs/specs/signed-interest.rst
+++ b/docs/specs/signed-interest.rst
@@ -114,7 +114,7 @@
 
 Recipients of a signed interest may further check the timestamp and the uniqueness of the
 signed interest (e.g., when the signed interest carries a command). In this case, a signed
-interest may be treated as invalid if :
+interest may be treated as invalid if:
 
 -  a valid signed Interest whose timestamp is **equal or later** than the timestamp of the
    received one has been received before.