docs: Escaped single backslash

docs/curl.1

@@ -1637,7 +1637,7 @@ without the domain, if there is a single domain and forest in your setup
 for example.
 
 To specify the domain name use either Down-Level Logon Name or UPN (User
-Principal Name) formats. For example, EXAMPLE\user and user@example.com
+Principal Name) formats. For example, EXAMPLE\\user and user@example.com
 respectively.
 
 If you use a Windows SSPI-enabled curl binary and perform Kerberos V5,

docs/libcurl/opts/CURLOPT_USERNAME.3

@@ -47,7 +47,7 @@ without the domain name should the server be part of a single domain and
 forest.
 
 To include the domain name use either Down-Level Logon Name or UPN (User
-Principal Name) formats. For example, EXAMPLE\user and user@example.com
+Principal Name) formats. For example, EXAMPLE\\user and user@example.com
 respectively.
 
 Some HTTP servers (on Windows) support inclusion of the domain for Basic

docs/libcurl/opts/CURLOPT_USERPWD.3

@@ -41,7 +41,7 @@ without the domain name should the server be part of a single domain and
 forest.
 
 To specify the domain name use either Down-Level Logon Name or UPN (User
-Principal Name) formats. For example, EXAMPLE\user and user@example.com
+Principal Name) formats. For example, EXAMPLE\\user and user@example.com
 respectively.
 
 Some HTTP servers (on Windows) support inclusion of the domain for Basic